In an internal combustion engine, noise is generated with the exhaust. Such exhaust noise is caused: by pressure waves generated by the difference between the pressure in the cylinder, and the pressure in the exhaust pipe at the moment when the exhaust valve is opened; by resonance in the pipe amplified and generated when the pressure waves reciprocate within the pipe; and by air current, i.e., the current of the exhaust.
An expansion type muffler, resonance type muffler, interference type muffler, sound absorption type muffler, and a muffler of a type combining the above mentioned types have already been proposed for eliminating exhaust noise.
In the expansion type muffler, an expansion chamber with a volume larger than that of the exhaust pipe is provided in the exhaust system so that the exhaust may be introduced into the expansion chamber, and the sound wave component may be attenuated in the chamber to silence the noise. It is known that this kind of expansion type muffler can silence the noise over a wide frequency range from a low sound range to a high sound range.
In such an expansion type muffler, there appears periodically a frequency passing band range in which noise is not silenced at all depending on the size of the expansion chamber. In view of the fact that a better silencing effect is obtained by eliminating such passing band, the frequency of this passing band has been shifted to a higher frequency range conventionally by dividing the expansion chamber or selecting the opening positions of the communicating pipes, i.e., providing the openings at different positions between the front and rear walls. However, there still exists the following problems.
In the event that the expansion chamber is divided, the volume per chamber would be decreased so that the back pressure to the engine would become somewhat higher than in a one-chamber type, and this exerts an adverse effect on the engine output. Further, the silencing effect of the low frequency range would be reduced due to the decrease of the chamber volume. In case the communicating pipe opening positions are provided at different positions relative to each other on the front and rear wall of the chamber, a void resonance of the passing band frequency would remain within the chamber to act as pressure waves on the muffler wall and would vibrate same to generate a wall vibration noise, such as so-called vibration sounds. This would also be the case if the interior of the expansion chamber is sectioned into a plurality of chambers, and the positions of the communicating pipes are differently selected on the respective chamber walls.
Therefore, there has been proposed a muffler wherein the partition plate, sectioning the interior of the expansion chamber, comprises a porous plate so that, by the presence of the perforations in the partition plate, a mode passing through the perforations and a mode generated on both sides of the partition plate may be produced. Due to such construction, the modes generated by the division are attenuated by the porous plate, the resonance caused in the sectioned chambers is prevented, the silencing effect within the muffler is increased, and the vibration sounds of the outer wall of the muffler by the resonant vibrations can be controlled.
Such muffler should have retained, by the presence of the porous partition plate, the features of a one-chamber type muffler and a divided-chamber type muffler to solve the problems as aforesaid. However, even in such muffler, the defect of the divided-chamber type remains. That is to say, because the interior of the expansion chamber is sectioned with the partition plate, the back pressure to the engine cannot help but become somewhat higher than in the one-chamber type so that the engine output is decreased.
The present invention effectively solves the above problems.